Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes than both incandescent bulbs and fluorescent tubes. In addition, the efficiency of conversion of electricity to light has now reached the same levels as obtained in fluorescent light fixtures.
In many applications, the cost of replacing a fluorescent tube and disposing of the defective tube is much greater than the cost of the tube itself. In commercial settings, the labor cost inherent in stocking and replacing tubes is high. In addition, the cost of disposal of fluorescent tubes further increases the cost of replacing the tubes, since fluorescent tubes utilize mercury to generate the underlying UV light that is then converted to visible light by the phosphors in the tubes, which are also hazardous materials that present a separate health hazard. Hence, care must be taken in handling the tubes and moving them to the disposal site to prevent breakage and the subsequent release of the toxic materials. Finally, the cost of the tubes themselves over the lifetime of the light fixture is also significant. Thus, there has been some interest in replacing existing fluorescent tubes with LED-based lighting elements.
Converting existing fluorescent fixtures to LED-based light sources presents a number of challenges. Existing fluorescent fixtures typically include an enclosure that holds one or more tubes and a ballast that converts the 50 or 60 cycle building power to the voltages and frequencies utilized by the tubes. A fluorescent tube typically has a startup phase in which a discharge is initiated in the gas. Once the discharge is established, a different voltage is applied to the fluorescent tube to maintain the discharge. The ballast provides the driving voltages and manages the startup phase. The voltages used to start and drive the fluorescent tube are typically AC voltages.
LEDs are typically driven with a DC voltage at a constant current. Hence, an LED light source typically requires a power supply that converts the conventional building power to a DC source that supplies a constant current to the LEDs in the light source. Accordingly, when an existing fluorescent tube light source is converted to an LED-based light source, all of the fluorescent tubes are typically removed and the fluorescent tube ballast is replaced with a LED power source.
This conversion can impose a significant cost and require a significant downtime for the lighting fixtures. The retired fluorescent tubes that are still functioning represent a significant capital investment that is lost when the fixture is replaced. In addition, specialized personnel are needed to remove the old ballast and install the new LED power supply. Accordingly, a switchover to LED-based tube replacements can require a significant level of organization and planning as well as cost.
The electrical conversion efficiency and lifetime of LEDs depend on the operating temperature of the LEDs. Increases in temperature lead to a loss in conversion efficiency and a lowering of the LED lifetime. Transferring the heat from the LEDs to the surrounding air is, hence, an important consideration in replacing existing fluorescent tubes with LED-based light sources. Typically, LED light sources in the power range of existing fluorescent tube light sources require a large heat transfer surface and good air circulation. If the fluorescent tubes are in a closed fixture that has poor air circulation, the transfer of the heat to the air presents additional challenges.